In particle beam lenses the aim is to combine good optical properties for the primary beam with good collecting properties for the secondary beams. In this case the starting point is conventional double-pole lenses consisting of an inner and an outer pole piece, wherein the lens-producing magnetic field is located largely within the physical lens. In this connection single-pole lenses have also been used, in which one pole is drawn out or the air gap which produces the magnetic field is disposed at the upper or lower end of the lens so that the magnetic field propagates largely outside the lens body.
A multi-pole lens is known from EP-A-0 133 016 which consists essentially of a row of two double-pole lenses of the type described above.
Starting from the prior art, the object of the invention is to make further improvements to the electron beam lens and particularly in its optical properties for the primary beam and in its collecting properties for the secondary beams.